1. Field of the Invention
The present invention relates to a semiconductor device having an insulated gate bipolar transistor and a control transistor which are formed on the same semiconductor substrate and, more particularly, to a protecting circuit for preventing latch up caused by a parasitic element generated when an insulated gate bipolar transistor and a control circuit are formed on the same semiconductor substrate by using a junction isolation technology.
2. Related Art
In a general semiconductor device, when a circuit element or a circuit region for a control circuit is formed on a semiconductor substrate on which an insulated gate bipolar transistor (to be referred to as an “IGBT” hereinafter) is formed, a parasitic element is formed. In particular, it is known that, when the circuit element is formed on the IGBT by using a junction isolation technology, a parasitic thyristor is formed on the substrate and the circuit region of the IGBT. There is a problem that when the parasitic thyristor is turned on (latched up), the semiconductor device itself can be broken down.
A countermeasure must be made by providing a protecting element to prevent the parasitic thyristor from being latched up. For example, a method of forming a protecting circuit for protecting latch up by a circuit by combining a resistor and a diode is known (see Reference 1). In a semiconductor device which has been developed before by the applicant of the present application, a means for preventing a thyristor from being latched up is constituted by a Schottky barrier diode (see Reference 2).
[Reference 1] “A Self-isolated intelligent IGBT for driving ignitioncoils”, International symposium on Power Semiconductor Drives. & Ics, 1998.
[Reference 2] Japanese Patent Laid-open Publication No. 2002-16254.
A means for preventing a parasitic thyristor from being latched up by using a Schottky barrier diode described in Reference 2 can efficiently achieve an object by using a protecting element which is smaller than that used in a means using a circuit obtained by combining a resistor and a diode. However, in contrast to this, since a Schottky barrier diode is inserted in series with a path extending an input terminal to an actual control circuit, a loss corresponding to a forward voltage drop of the diode is generated in a signal transmitted to the control circuit. In particular, generation of a loss in a signal which can be transmitted to the IGBT when a voltage to be input is low makes it impossible to supply a sufficient voltage to the gate of the IGBT. As a result, a sufficient collector current cannot be obtained.